1. Field of the Invention
The present invention relates to a technique for driving a capacitive light-emitting element; e.g., an organic electroluminescence (EL) element, to emit light. Particularly, the invention relates to an apparatus and method for driving a luminescent display panel which suppresses occurrence of crosstalk illumination of EL elements and can offer a suitable luminous brightness characteristic by means of appropriately controlling a reverse bias voltage to be applied to cathode scanning lines in a non-luminous state, as required, when a display panel having a plurality of organic EL elements arranged thereon is driven.
2. Description of the Related Art
An organic EL display has already been put into actual use in some quarters as a display which serves as an alternative to a liquid-crystal display and enables realization of low power consumption, high display quality, and a lower profile. An underlying backdrop to this is that the efficiency and life of an EL display have been improved to a practical level, by use of an organic compound—which can be expected to yield a superior light-emitting characteristic—for a light-emitting layer which is made of EL elements and is to be used for an EL display.
The organic EL element can be electrically represented as an equivalent circuit as shown in FIG. 4. The organic EL element can be replaced with a configuration consisting of a diode component E and a parasitic capacitance component Cp connected in parallel with the diode component E. The organic EL element is considered to be a capacitive light-emitting element. Illumination is considered to be effected in the following manner. When a luminous drive voltage is applied to the organic EL element, electric charges corresponding to the electrical capacitance of the element flow into and are stored in electrodes as a displacement current. Subsequently, when the electric charges have exceeded a given voltage inherent to the element (an emission threshold value=Vth), an electric current starts flowing from the electrode (i.e., the anode of the diode component E) into an organic layer constituting the light-emission layer. The light-emission layer illuminates at an intensity proportional to the electric current.
FIGS. 5A to 5C show a static light-emission characteristic of such an organic EL element. As shown in FIG. 5A, when a drive voltage (V) exceeds the emission threshold-value voltage (Vth), an electric current (I) abruptly flows into the organic EL element, whereupon the EL element illuminates. In other words, if the applied drive voltage is lower than the emission threshold-value voltage, a drive current does not flow into the EL element after recharging of the parasitic capacitance, and hence the EL element does not illuminate. As shown in FIG. 5B, within a light-generative domain in which the drive voltage (V) exceeds the emission threshold-value voltage, the EL element has a characteristic of illuminating at luminance (L) substantially proportional to the drive current (I). Consequently, as shown in FIG. 5C, within the light-generative domain in which the drive voltage (V) exceeds the threshold-value voltage the EL element has a luminance characteristic such that light-emission luminance of the EL element becomes greater as the value of the voltage (V) applied to the same increases.
The organic EL element has a characteristic of the physical properties thereof changing with long-term use and the resistance thereof becoming greater. As shown in FIG. 5A, with lapse of operating time a V—I characteristic of the organic EL element changes in the direction indicated by the arrowhead (i.e., assumes a characteristic designated by broken lines). Consequently, the luminance characteristic of the EL element also deteriorates.
The luminance characteristic of the organic EL element is also known to change roughly in the manner as indicated by broken lines in FIG. 5C according to an ambient temperature. Specifically, within the light-generative domain in which the drive voltage (V) exceeds the emission threshold-value voltage, the EL element has a characteristic of light-emission luminance (L) thereof becoming greater as the voltage (V) applied to the element becoming greater. However, the emission threshold-value voltage becomes lower as ambient temperature rises. Consequently, when heated to a higher temperature, the EL element becomes able to emit light at a lower applied voltage. Further, in relation to luminance, the EL element has temperature dependence of illuminating brightly at a high temperature and illuminating dimly at a low temperature even when a light-generative voltage has been applied to the EL element.
As a method of driving a display panel constituted by arranging a plurality of organic EL elements, a simple matrix drive system is applicable. FIG. 6 shows an example of a simple matrix display panel and a drive unit therefor. A method of driving organic EL elements of the simple matrix drive system includes two methods; that is, a method of scanning cathode lines and driving anode lines, and a method of scanning anode lines and driving cathode lines. The configuration shown in FIG. 6 is associated with the former method; that is, a method of scanning cathode lines and driving anode lines. More specifically, anode lines A1 through An serving as “n” drive lines are arranged in the vertical direction, and cathode lines B1 through Bm serving as “m” scanning lines are arranged in the horizontal direction. Organic EL elements (OEL) assigned diode symbols are disposed at respective intersections between the cathode and anode lines (a total of “n”×“m”).
The EL elements constituting pixels are arranged in a grid pattern. The EL elements constituting pixels are provided in corresponding intersections between the positive drive lines A1 through An laid vertically and the cathode scanning lines B1 through Bm laid horizontally. Each of the EL elements is connected at one end (e.g., an anode terminal of the diode component E in the previously-described equivalent circuit) to an anode drive line and at the other end (e.g., a cathode terminal of the diode component E in the equivalent circuit) to a cathode scanning line. The anode drive line is connected to and driven by an anode line drive circuit 2, and the cathode scanning line is connected to and driven by a cathode line scanning circuit 3.
The cathode line scanning circuit 3 is equipped with scanning switches SY1 through Sym corresponding to the respective cathode scanning lines B1 through Bm. The cathode line scanning circuit 3 operates so as to connect, to a corresponding cathode scanning line, either a reverse bias voltage (VM) output from a reverse bias voltage generation circuit 5 for preventing occurrence of crosstalk illumination, or a ground potential serving as a reference potential. Further, the anode line drive circuit 2 is equipped with constant current circuits I1 through In and drive switches SX1 through SXn, wherein the constant current circuits I1 through In act as constant current sources for supplying drive currents to the respective EL elements through corresponding anode drive lines.
The drive switches SX1 through SXn act so as to connect to corresponding anode lines either ground potential or the electric current output from the constant current circuits I1 through In. Hence, the drive switches SX1 through SXn are connected to the constant-current circuits I1 through In, whereby the electric currents output from the constant current circuit I1 through In are supplied to the respective EL elements arranged so as to correspond to the cathode scanning lines.
A drive source, such as a constant voltage circuit, may be used in place of the constant current circuit. The current/luminance characteristic of the EL element is stable, whereas a voltage/luminance characteristic of the same is unstable. In addition, a constant current circuit is generally used as a drive source, as shown in FIG. 6, for reasons of preventing deterioration of the element, which would otherwise be caused by excessively high current.
The anode line drive circuit 2 and the cathode line scanning circuit 3 are connected to an illumination control circuit 4 by way of control buses. On the basis of an image signal which is to be supplied to the illumination control circuit 4 and to be displayed, the scanning switches SY1 through Sym and the drive switches SX1 through SXn are actuated. On the basis of the image signal, the cathode scanning lines are set to a reference potential at predetermined cycles, and the constant current circuit is connected to a desired anode line. As a result, the respective light-emitting elements are selectively illuminated, whereupon an image is reproduced on the display panel 1 in accordance with the image signal.
A DC output (a drive voltage=VCOM) output from a booster circuit 6 constituted of a DC-DC converter is supplied to the respective constant current circuits I1 through In in the anode line drive circuit 2. The booster circuit 6 which is constituted of a DC-DC converter and will be described later produces a d.c. output through pulse width modulation (PWM) control. Alternatively, pulse frequency modulation (PWF) may be utilized.
The DC-DC converter is configured such that an n-p-n transistor Q1 serving as a switching element is activated at a predetermined duty cycle by means of a PWM waveform output from the switching regulator circuit 11. By means of activation of the transistor Q1, the electric power energy output from a DC voltage source 12 is accumulated in an inductor L1. In association with deactivation of the transistor Q1, the electric power energy accumulated in the inductor is stored in a capacitor Cl via a diode D1. Through repeated activation and deactivation of the transistor Q1, a boosted DC output can be obtained as a terminal voltage of the capacitor C1.
The DC output voltage is divided by a parallel circuit constituted of a resistor R3 and a thermistor TH1 for temperature compensation and at a junction between a resistor R1 and a resistor R2 connected in series with the parallel circuit. The thus-divided output voltage is supplied to an error amplifier 14 in the switching regulator circuit 11, the amplifier being constituted of an operational amplifier. The error amplifier 14 compares the output voltage with a reference voltage Vref. A comparison output (i.e., error output) is supplied to the PWM circuit 15, thereby controlling the duty cycle of a signal wave output from an oscillator 16. In this way, the DC-DC converter is subjected to feedback control such that the output voltage is maintained at a predetermined constant voltage.
By means of the configuration shown in FIG. 6, the thermistor TH1 is inserted into the feedback system so as to provide feedback to the error amplifier 14. The output voltage Vout produced by the DC-DC converter 6 is adjusted by means of the temperature characteristic of the thermistor TH1. Eventually, the reverse bias voltage VM—which is produced by means of dividing the output voltage Vout and will be described later—is varied in accordance with ambient temperature. Here, the output voltage Vout produced by the DC-DC converter 6 can be expressed as follows. In the following equation, “TH1//R3” denotes a parallel combined resistance value produced from the resistance of the thermistor TH1 and that of the resistor R3.Vout=Vrefx[(R1+R2+TH1//R3)/R1]
The reverse bias voltage generation circuit 5 utilized for preventing occurrence of the foregoing crosstalk illumination is constituted of a potential dividing circuit for dividing the output voltage Vout. The potential dividing circuit is constituted of resistors R4, R5 and an n-p-n transistor Q2 serving as an emitter follower. Therefore, when a base-emitter voltage in the transistor Q2 is taken as Vbe, the reverse bias voltage VM produced by the potential dividing circuit can be approximated as follows.VM=Vout×[R5/(R4+R5)]−Vbe
In the foregoing configuration, the illumination control circuit 4 controls the drive switches SX1 through SXn in the anode line drive circuit 2 in accordance with an image signal while scanning the cathode lines B1 through Bm in the cathode line scanning circuit 3 at a predetermined cycle, thus selectively connecting the constant-current circuits I1 through In to the respective anode drive lines A1 through An. At this time, the reverse bias voltage VM output from the reverse bias voltage generation circuit 5 is applied to the cathode lines in a non-scanning state. As a result, the EL elements connected to the interconnections between the anode line being driven and the cathode lines not selected for scanning operate so as to prevent occurrence of crosstalk illumination.
As mentioned previously, the organic EL element has the parasitic capacitance Cp. For instance, there is taken as an example a case where one anode drive line is connected to tens of EL elements, from the viewpoint of the anode drive line having a combined capacitance—which is greater than each parasitic capacitance by an order of magnitude—being connected to the anode drive line as load capacitance.
Consequently, the electric current output from the anode drive line at the leading end of a scanning period is spent in recharging the load capacitance. If the load capacitance is recharged until the emission threshold-value voltage of the EL element is sufficiently exceeded, a time lag will arise. This eventually presents a problem of a delay arising in start-up of the EL element. As mentioned previously, particularly in the case where the constant current sources II through In are used as a drive source, the constant current sources correspond to high-impedance output circuits in terms of principle of operation. Hence, a limitation is imposed on an electric current, thereby inducing a noticeable delay in the rise and illumination of the EL element.
The drive circuit of this type usually adopts a cathode resetting method. The cathode resetting method is described in, e.g., Japanese Patent Application Laid-Open No. 2320074/1997. When one scanning line has been switched to another scanning line, the method acts so as to speed up the rise and illumination of an EL element which is to be driven and illuminated by the current scanning line.
The drive switches SX1 through SXn provided in the anode line drive circuit 2 are connected to either the constant current sources I1 through In or the ground potential. When the switches SX1 through SXn are connected to the ground potential, the drive anode lines are set to the ground potential. Consequently, the cathode resetting method can be realized by utilization of the drive switches SX1 through SXn.
FIGS. 7A to 7D are illustrations for describing a cathode resetting operation. For instance, there is shown that a shift arises from a state in which an EL element E11 connected to the first anode drive line A1 is driven and activated to another state in which an EL element E12 connected to the first anode drive line A1 is driven and illuminated. In FIGS. 7A through 7D, an EL element to be driven and illuminated is depicted as a diode symbol, and the other EL elements are depicted as symbol of capacitors serving as parasitic capacitance.
FIG. 7A shows a state in which a cathode resetting operation is performed and in which the EL element E11 is illuminated as a result of a cathode line B1 having been scanned. The EL element E2 is to be illuminated through next scanning operation. However, before illumination of the EL element E12, the anode drive line A1 and all cathode scanning lines are reset to the ground potential as shown in FIG. 7B, thereby discharging all electric charges from the respective EL elements. To this end, the scanning switches SY1 through SYm are connected to ground, and the drive switch SX1 is connected to ground. In order to illuminate the EL element E12, a cathode scanning line B2 is scanned. In other words, the cathode scanning line B1 is grounded, and the remaining cathode scanning lines are given the reverse bias voltage VM. At this time, the drive switch SX1 is switched to the constant current source I1.
At the time of resetting operation, electric charges, which correspond to parasitic capacitance of the respective EL elements, are discharged. At this moment, as shown in FIG. 7C, the parasitic capacitance of the EL elements, except the EL element E12 which is to be illuminated next, is charged with the reverse bias voltage VM in a reverse direction as indicated by an arrow. This charging current flows into the EL element E12 to be illuminated next, via the anode drive line A1, thereby charging the parasitic capacitance of the EL element E12. At this time, as mentioned previously, the constant current source I1 connected to the drive line A1 in principle corresponds to a high-impedance output circuit. Hence, the constant current source I1 does not affect the flow of the charging current.
Provided that, for example, 64 EL elements are provided in the drive line A1 and that the reverse bias voltage is, e.g., 10(V), the potential V (A1) of the anode drive line A1 momentarily rises to a potential defined by Eq. 3 provided below through recharging operation, because line impedance of the panel is negligibly small. For instance, in the case of a display panel having outer dimensions of about 100 mm×25 mm (256×64 dots), a rise in the potential of the anode drive line is completed at about 1 μsec.V(A1)=(VM×63+0V×1)/64=9.84V
By means of the drive current which flows through the drive line A1 and originates from the constant current source I1, the EL element E12 is brought into an illuminating state, as shown in FIG. 7D. As has been described, the cathode resetting method acts so as to instantaneously increase the forward voltage of the next EL element be driven and illuminated, by utilization of parasitic capacitance of EL elements, which would originally hinder operation thereof, and a reverse bias voltage for preventing occurrence of crosstalk illumination.
When the cathode resetting method set forth is utilized, the forward voltage of an EL element to be driven and illuminated through the next scanning operation is started momentarily, and the EL element is driven and illuminated upon receipt of a drive current from the constant current source. Consequently, if the value of the reverse bias voltage VM is set higher, occurrence of crosstalk illumination can be effectively inhibited. Further, an initial charging voltage—which is a forward voltage to be supplied to an EL element to be illuminated through the next scanning operation—increases correspondingly. Therefore, at first glance the cathode resetting method is considered to be preferable. However, if the value of the reverse bias voltage VM is set excessively high, a so-called leakage phenomenon will arise, thereby deteriorating the display grade of the display panel. For this reason, in relation to a related-art drive circuit of this type, the reverse bias voltage VM is set to a fixed voltage close to the forward voltage Vf of the EL element.
As has been described by reference to FIG. 5A, the EL element of this type involves a problem of a forward voltage increasing with time. Further, as has been described by reference to FIG. 5C, the EL element of this type also involves a problem of a forward voltage varying in accordance with ambient temperature. For instance, in a case where a rise has arisen in a forward voltage after long-term use, a discrepancy gradually develops between a voltage VM with which an EL element is initially charged immediately before a scanning operation and the forward voltage Vf of the EL element, because the reverse bias voltage VM is a fixed voltage. Consequently, a delay arises in the time at which an EL element starts illuminating by means of an initial charging operation using the fixed reverse bias voltage VM, along with a problem of the quantity of illumination of the EL element gradually decreasing. In other words, a period during which a predetermined quantity of illumination of the EL element can be ensured is shortened, thereby turning into another problem of the life of the EL element becoming essentially short.
In addition to the changes with time and temperature dependence set forth, variations in film growth (deposition) treatment performed at the time of producing an EL element induce variations in the forward voltage of the EL element of this type. The EL element of this type involves a problem of a forward voltage changing according to the color of illumination, such as red (R) illumination, green (G) illumination, or blue (B) illumination. Eventually, variations arise in the light-emission luminance of the EL element.
Even in a case where a generation circuit constituted of a resistive divider and an emitter follower such as that shown in FIG. 6 is adopted as means for generating a reverse bias voltage VM, if the forward voltage Vf is higher than the reverse bias voltage VM, there arises a phenomenon of variations arising in an electric current which flows through an emitter-follower resistor via parasitic capacitance of respective EL elements in a non-scanning line in accordance with the number of elements illuminating in the display panel and with illumination luminance of the same. Therefore, the reverse bias voltage VM fluctuates, and variations arise in a potential difference between the reverse bias voltage VM and the forward voltage Vf of the element, eventually inducing variations in the illumination luminance of the EL element.
As shown in FIG. 6, even if the thermistor TH1 is used to consequently subject the reverse bias voltage VM to temperature compensation, the thermistor TH1 responds to temperature compensation slowly. Further, a temperature compensation curve does not necessarily match the characteristic of the EL element. For these reasons, difficulty is encountered in achieving a satisfactory compensation characteristic. Under ideal arrangement of the thermistor, the thermistor is brought into thermally intimate contact with a display panel. However, in reality, adoption of such a configuration is difficult, thereby posing difficulty in arranging and designing a thermistor.